In the scrap metals recycling industry there is a lack of an environmentally clean low cost technology to reliably segregate post-consumer metal scrap into its various metal constituents. Current practice for sorting aluminum metals from other nonmagnetic metals derived from scrap sources such as automobile shredders is to either sort by hand labor or to introduce the mixed metals into a liquid heavy media where the aluminum floats and the heavier nonmagnetic metals sink. Hand labor is far too slow and very expensive. The heavy media process is capital intensive, has high operating costs, and uses a water slurry mixed with chemicals to raise the specific gravity of the liquid to a value above that of aluminum (2.7 gm/cc). The liquid media requires treatment in a wastewater treatment facility. The resulting sludge composed of grease, oil, dirt, and chemicals poses significant disposal issues as do water discharges. Additionally to be cost effective the heavy media process requires a large installation and is normally deployed as a regional facility. This requires the producers of scrap to ship their metals to this regional facility for separation before the metal products can be shipped to market, whereas if sorting could be accomplished locally the scrap producers could ship directly to market. Elimination of the extra shipping requirement would improve the economics of recycling and remove the burden on our environment caused by the shipping of hundreds of thousands of tons of scrap metals annually to regional heavy media plants.
There have been recent efforts to develop dry environmentally friendly techniques to sort low atomic number light weight metals and alloys such as magnesium (atomic number Z=12) and aluminum (Z=13) and their alloys from higher atomic number heavier metals such as iron (Z=26), copper (z=29), and zinc (Z =30) and their alloys. One method is to acquire and analyze x-ray fluorescence spectra derived from metals by irradiating metals with excitation x-rays, measuring the resulting x-ray fluorescence emitted from the metals, utilizing spectral information developed from the measurements to identify composition of the metals, and to mechanically sort the metals according to their compositions. This method is exemplified by U.S. Pat. Nos. 6,266,390, 6,519,315, and 6,888,917. Low Z material does not lend itself well to x-ray fluorescence analysis since x-ray photons fluoresced from low Z materials are at low yield and are low energy (˜1-2 kev). Because they are low energy they are easily absorbed in air before reaching the detection system. This method also, by nature of the detection system, requires a significant time interval to build and analyze spectral information for each piece of material analyzed. Consequently systems that operate according to this method are limited in throughput rate of materials. For high throughput rates it is desired to have a faster acting analyses system in order to process materials faster and at greater volumes.
Another effort is described in Patent Application Publication No. US2004/0066890 wherein is discussed a process of irradiating materials by x-ray radiation, measuring x-ray transmission values through materials at two different energy levels, and using these measurements to determine the thickness and composition of a material. However, that publication does not reveal how such determinations can be accomplished. That dual energy system, as described, discusses utilizing undisclosed image processing techniques and appears similar to standard security x-ray scanners, such as those used at security checkpoints in airports, which utilize x-ray measurements at two different energy levels to measure thickness and material composition and present on a computer monitor screen a complex image for human visual inspection which is graphically encoded by image intensity and false color mapping to represent thickness and material composition (as described by security x-ray scanner vendor Smith's Heimann). Some such x-ray scanners utilize a physically stacked dual energy x-ray detector array to measure x-ray transmission values through materials over two energy ranges, the fundamental features of which are described by GE Medical Systems in their U.S. Pat. Nos. 4,626,688 and RE 37,536. A stacked dual energy detector utilizes a physical geometry of having a lower energy detector sandwiched with a higher energy detector with a filter, typically a metal layer such as copper, interposed between the two detectors. X-rays to be measured first enter the detector stack into the lower energy detector. Lower energy photons are absorbed by the lower energy detector as they are measured. Mid-energy and higher energy photons pass through the lower energy detector. Mid-energy photons are absorbed in the filter layer between the two detectors while higher energy photons pass through the filter layer and are measured by the higher energy detector at the back of the stack. Other x-ray scanners utilize other types of dual energy detector arrangements, such as side-by-side arrays, examples of which are disclosed in U.S. Pat. Nos. 5,841,832 and 5,841,833.
Still another effort utilizes spectral analysis of plasma evaporated off the surface of metal samples induced by momentarily striking the metals with a focused high power laser beam. This method, referred to as Laser Induced Breakdown Spectroscopy or LIBS, reportedly has been practiced in the U.S. by a metals processing company and is detailed in U.S. Pat. No. 6,545,240 B2. The LIBS process for sorting of metals as they are conveyed in volume through a processing line involves a high level of complexity due in part to requirements to rapidly steer a laser beam to small target points from sample to sample for repeated bursts of laser light and to correspondingly steer spectral acquisition optics from sample to sample in coincidence with the laser beam. This method is very complex and costly.
In sorting of many materials, such as nonferrous automobile scrap, it is very advantageous to be able to sort lighter weight materials (such as aluminum and its alloys) from heavier weight materials (such as iron, copper, and zinc and their alloys). To accomplish such a sort it is not necessary to determine both thickness and composition as the method of US2004/0066890 claims to do and it is not necessary to use complex image processing techniques of US2004/0066890 and as practiced using security x-ray scanners. Instead a determination of relative composition, such as relative average atomic number (Z), suffices to make a very valuable sort of the materials. Determination of relative composition, such as in discriminating high Z materials from low Z materials, is simpler for a detection system to accomplish than is determination of thickness and composition which can require high precision detector signals to be able to discern fine differences in measurements from sample to sample, maintenance of comprehensive detection system calibrations, and use of complex pattern matching algorithms such as those used by human visual inspectors in interpreting processed images produced by security x-ray scanners. At this time it has not been technically possible to duplicate by computerized algorithms the complex visual pattern matching skills used by humans in interpreting processed images produced by dual energy x-ray scanner security systems.